The subtalar joint axis palpation technique, part 1: validating a clinical mechanical model

Background: Locating the position of the subtalar joint axis can be a predictive clinical variable in biomechanical analysis and a valuable tool in the design of functional foot orthoses. Before testing Kirby's palpation technique to locate the subtalar joint axis in cadavers, it was important to develop and test the experimental methods in a mechanical model in which the exact location of the hinge joint can be controlled. Methods: Four testers determined the hinge joint location and moved it through its range of motion, capturing the movement of the joint axis using a kinematic model. The joint axis location was determined and validated by comparing the actual hinge joint location on the mechanical model with the location determined by the palpation technique described by Kirby in 1987 and the location determined by the helical joint axis method using three-dimensional kinematic data. Results: The overall angles result in mean slopes and intersections of 87 degrees and 92 mm, 86 degrees and 97 mm, 85 degrees and 92 mm, and 88 degrees and 91 mm for testers 1, 2, 3, and 4, respectively. Testers 1 and 3 were able to determine the location to 1 degrees and 1 mm accuracy, tester 2 to 0 degrees and 4 mm, and tester 4 to 2 degrees and 2 mm compared with the kinematic data. Conclusions: The technique of determining the points of no rotation as described by Kirby could be validated by using a three-dimensional kinematic model to determine the helical axis